Active Materials

Materials expand themselves depending on the temperature or
pressure; some materials deform themselves if an electromagnetic
field is applied to them. These materials are called active or smart materials.
Ideally, smart materials can change their color, stiffness, transparency,
and shape responding to a signal. Every child knows an example of
smart material: the monstrous troll, when exposed to the sunlight, becomes
stone. Enchanted princess are transformed similarly, responding to the
proper spell.

Variation of shape of a material can be magnified if differently
expanded materials are assembled in a structure. The Classical example
of this magnification is a bi-metallic plate. These plates that has been
used in English chronometers starting from XVIII century to compensate
for the high temperature in lower latitudes. Today, structures from active
materials are used in switches, sensors, actuators, etc.